Photophysical behavior of systematically substituted (di-2-pyridylaminomethyl) benzene ligands and its Re(I) complexesA combined experimental and theoretical approach

Article abstract of DOI:10.1016/j.jphotochem.2017.03.031

The photophysical behavior of a series of flexible di-2-pyridylaminomethyl substituted ligands systematically substituted on a rigid benzene core and their corresponding mono-metallic Re(I) complexes have been investigated by steady state and time-resolved fluorescence spectroscopy in different composition of DMSO–water binary solvent mixtures. Unusual fluorescence properties in lower DMSO content (X_DMSO?=?0.1–0.3) solvent mixtures is consistent with fascinating property of DMSO, which is known to perturb the hydrogen bonding ability of water with the solute. Spin allowed inter-ligand π–π^* transition is more apparent in (1, 3) substituted systems. The calculated spectroscopic parameters of the complexes are significantly different from the ligands, mainly due to ligand to metal charge transfer. The experimental observations are in very good agreement with the theoretical results obtained at B3LYP/6-31G(d,p)/LANL2DZ level of density functional theory (DFT) calculation. Natural bond orbital (NBO) analysis and examination of frontier molecular orbitals reveal that the basic architecture of the symmetrically substituted multi-chromophoric ligand can induce excitation energy hopping, similar to an artificial antenna system.

Full text of DOI:10.1016/j.jphotochem.2017.03.031

Accepted Manuscript
Title: Photophysical behavior of systematically substituted
(di-2-pyridylaminomethyl) benzene ligands and its Re(l)
complexes: A combined experimental and theoretical
approach
Author: Mostofa Ataur Rohman Dipankar Sutradhar S.
Sangilipandi K. Mohan Rao Asit K. Chandra Sivaprasad Mitra
PII:
S1010-6030(17)30090-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.03.031
JPC 10582
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
19-1-2017
21-3-2017
22-3-2017
Please cite this article as: M.A. Rohman, D. Sutradhar, S. Sangilipandi, K.M.
Rao, A.K. Chandra, S. Mitra, Photophysical behavior of systematically substituted
(di-2-pyridylaminomethyl) benzene ligands and its Re(l) complexes: A combined
experimental and theoretical approach, Journal of Photochemistry and Photobiology
A: Chemistry (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.03.031
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Photophysical
behavior
of
systematically
substituted
(di-2-
pyridylaminomethyl) benzene ligands and its Re(l) complexes: A combined
experimental and theoretical approach
Mostofa Ataur Rohman, Dipankar Sutradhar, S. Sangilipandi, K. Mohan Rao, Asit K.
Chandra, Sivaprasad Mitra*
Centre for Advanced Studies, Department of Chemistry, North-Eastern Hill University, Shillong
– 793022, India
*To whom correspondence should be addressed. Phone: (91)-364-2722634. Fax: (91)-364-2550076. E-
mail: smitranehu@gmail.com, smitra@nehu.ac.in
1
Page 1 of 46

Abstract
The photophysical behavior of a series of flexible di-2-pyridylaminomethyl substituted
ligands systematically substituted on a rigid benzene core and their corresponding mono-metallic
Re (I) complexes have been investigated by steady state and time-resolved fluorescence
spectroscopy in different composition of DMSO-water binary solvent mixtures. Unusual
fluorescence properties in lower DMSO content (XDMSO = 0.1~0.3) solvent mixtures is consistent
with fascinating property of DMSO, which is known to perturb the hydrogen bonding ability of
water with the solute. Spin allowed inter-ligand ππ* transition is more apparent in (1, 3)
substituted systems. The calculated spectroscopic parameters of the complexes are significantly
different from the ligands, mainly due to ligand to metal charge transfer. The experimental
observations are in very good agreement with the theoretical results obtained at B3LYP/6-
31G(d,p)/LANL2DZ level of density functional theory (DFT) calculation. Natural bond orbital
(NBO) analysis and examination of frontier molecular orbitals reveal that the basic architecture
of the symmetrically substituted multi-chromophoric ligand can induce excitation energy
hopping, similar to an artificial antenna system.
Key-words: (2-pyridyl-aminomethyl) benzene Re(I) complex; time-resolved fluorescence; DFT
calculation; energy transfer and trapping; antenna system.
2
Page 2 of 46

1. Introduction
Controlled synthesis and characterization of three dimensional molecular architectures
with suitable core and peripheral subunits are of tremendous interest in the fields of guest-host
chemistry, analytical chemistry, optoelectronics, catalysis, biology, and medicine etc. In contrast
to the other macromolecules, the size, shape, and functionality of the giant dendritic structures
can be defined at the molecular level. The photophysical and functional behavior of the core and
peripheral ligands are often suitably chosen to fabricate target designed molecular systems [1-3].
Further, the photophysical and photochemical properties of diamine complexes with d6
transition metal (TM) ions, such as rhenium (I), ruthenium (II) and osmium (II) have drawn
considerable attention due to their versatile applications in basic science and industry [4]. In
general, the advantages of TM-complexes are due to their specific photophysical properties. Due
to the presence of strong spin-orbit coupling through the coordinating metal [5, 6], TM-
complexes with suitable organic ligands often show intense phosphorescence at ambient
temperature [7 – 9], leading to their applications in organic light emitting diodes (OLEDs) [10]
or in other fields such as solar energy conversion, catalysis, as well as medical chemistry [11 –
14]. Typically, the excited state decay lifetimes vary within the range of tens of microseconds to
several hundred nanoseconds. This is about a few orders of magnitude longer than the typical
nanosecond fluorescence lifetime of organic ligands and many orders of magnitude shorter than
the corresponding phosphorescence lifetime (milliseconds to seconds) [15]. The situation is
particularly suitable for using the signal output from a TM-based probe in biological staining
application [16 – 17]. Re (I) tricarbonyl diimine complexes of the type Re(CO)3(N^N)L, where
N^N is an aromatic diimine and L is a monodentate ancillary ligand, often show intense and
long-lived luminescence typically in the green to orange spectral region. In most cases, the
3
Page 3 of 46

emission has been assigned to the lowest triplet state of a metal-to-ligand charge transfer
(3MLCT) character resulting from a d(Re) → π*(diimine) electronic transition [18 – 22]. The
photophysical and photochemical properties of the Re(I) tricarbonyl complexes can be tuned by
varying their ligand structures. Di-2-pyridylamine is a planar bidentate ligand with powerful
binding abilities towards Re (I) metal ion and it can be easily modified by the introduction of
substituent groups at its different positions. Also, the introduction of substituent groups such as
electron withdrawing or -donating moieties is helpful to tune the energy levels of the novel 2,2′-
bipyridine ligand [23 – 24].
In this paper, we report the photophysical behavior of a series of bis, tris, tetrakis-(di-2-
pyridylaminomethyl)benzene ligands (L1 – L5) and their mononuclear Re(I) complexes (C1 –
C5) (structures given in chart 1) studied by steady state and time-resolved fluorescence
spectroscopy in conjunction with density functional theory (DFT) calculation. As the solubility
of all the compounds is relatively poor in aqueous medium, the basic photophysical studies were
done in DMSO solution as well as in a series of DMSO-water mixtures by stepwise addition of
water (upto 98% v/v corresponding to Xwater = 0.99) for all the systems. It is to be noted here that
the possibility of any aggregate formation in the higher water content solution mixture is
eliminated by dynamic light scattering (DLS) experiment as well as transmission electron
microscope (TEM) measurement. A series of different spectroscopic experiments are already
reported to reveal the exceptional behavior of DMSO-water mixed solvent system [25 – 27].
Also, the mixture of DMSO and water is known to have extensive biological importance [28 –
32]. The physico-chemical properties of these mixtures deviate strongly from the continuum
behavior and differ to a large extent from that expected in the additive rule [33]. Therefore, to
understand the fluorescence behavior of systematically substituted di-2-pyridylaminomethyl-
4
Page 4 of 46

benzene ligands and the corresponding Re (I) complexes seems to be interesting from a
fundamental point of view, particularly in the mixture with varying DMSO-water composition.
2. Experimental details
2.1. Materials
2.1.1. Ligands: Synthesis and characterization of the ligands (L1 – L5) were done according to
literature reports published elsewhere [34, 35]. In this section, we present the results in brief.
1,2-Bis(di-2-pyridylaminomethyl)benzene (L1). Di-2 pyridylamine (1.00 g, 5.84 mmol) and
potassium hydroxide (1.33 g, 23.7 mmol) were stirred in DMSO (5 mL) for 1 h; 1,2 bis
(bromomethyl)benzene (0.70 g, 2.65 mmol) was added and the reaction mixture was stirred for a
further 40 h. Water was then added until the solution turned cloudy. The yellow precipitate that
ο
formed was isolated and recrystallized from an ethyl acetate– petroleum ether (bp 30–60 C)
solution to yield yellow crystals. Yield, 0.29 g (25%); mp 144–146 οC. Found: C, 75.37; H, 5.38;
N, 19.04. Calc. for C28H24N6: C, 75.65; H, 5.44; N, 18.91%. 1H NMR (500 MHz, CDCl3, 296 K)
d 8.32 (4H, d, py), 7.53 (4H, t, py), 7.27 (2H, d, ph), 7.22 (4H, d, py), 7.04 (2H, t, ph), 6.86 (4H,
13
dd, py), 5.64 (4H, s, CH2). C NMR (125.76 MHz, CDCl3, 296 K) d 157.1 (C5), 148.2 (C1),
137.2 (C7), 136.1 (C3), 126.5, 126.4 (C8, C9), 117.2 (C2), 114.6 (C4), 48.6 (C6). MS (HR-ESI):
m/z 444.2079; C28H24N6 requires 444.2063.
1,3-Bis(di-2-pyridylaminomethyl)benzene (L2). Di-2pyridylamine (1.00 g, 5.84 mmol) and
potassium hydroxide (1.33 g, 23.7 mmol) were stirred in DMF (30 mL) at 40 οC for 20 min. 1,3-
ο
bis (bromomethyl)benzene (0.699 g, 2.65 mmol) was added and stirring was continued at 40 C
for a further 40 h. The solvent was removed under reduced pressure and the residue partitioned
between dichloromethane (200 mL) and water (150 mL). The organic layer was washed twice
with water (2 × 150 mL) and then the combined aqueous phases washed with dichloromethane (2
5
Page 5 of 46

× 100 mL). The combined dichloromethane layers were dried (Na2SO4) and then evaporated,
resulting in a pale yellow solid. This was recrystallized from acetone–water yielding 980 mg
ο
(83%) of a yellow crystalline solid; mp 133–134 C. Found: C, 75.46; H, 5.49; N, 19.03. Calc.
for C28H24N6: C, 75.65; H, 5.44; N, 18.91%. 1H NMR (500 MHz, CDCl3, 296 K) d 8.25 (4H, d,
py), 7.53 (4H, t, py), 7.27 (2H, d, ph), 7.22 (4H, d, py), 7.04 (2H, t, py), 6.86 (4H, t, py), 5.64
13
(4H, s, CH2). C NMR (500 MHz, CDCl3, 296 K) d 157.0 (C5), 148.1 (C1), 139.3 (C7), 137.1
(C3), 128.4 (C9), 125.3 (C10), 125.1 (C8), 117.2 (C2), 114.5 (C4), 51.20 (C6).
1,4-Bis(di-2-pyridylaminomethyl)benzene (L3). Di-2-pyridylamine (1.00 g, 5.84 mmol) and
potassium hydroxide (1.33 g, 23.7 mmol) were stirred in DMF (30 mL) at 40 οC for 20 min.1,3-
ο
Bis(bromomethyl)benzene (0.699 g, 2.65 mmol) was added and stirring was continued at 40 C
for a further 40 h. The solvent was removed under reduced pressure and the residue partitioned
between dichloromethane (200 mL) and water (150 mL). The organic layer was washed twice
with water (2 × 150 mL) and then the combined aqueous phases washed with dichloromethane (2
× 100 mL). The combined dichloromethane layers were dried (Na2SO4) and then evaporated,
resulting in a pale yellow solid. This was recrystallized from acetone–water yielding 780 mg
ο
(66%) as a yellow powder; mp 181–183 C. Found: C, 75.47; H, 5.44; N, 19.01. Calc. for
C28H24N6: C, 75.65; H, 5.44; N, 18.91%. 1HNMR (500 MHz, CDCl3, 296 K) d 8.28 (4H, d, py),
13
7.47 (4H, t, py), 7.22 (4H, s, ph), 7.13 (4H, d, py), 6.82 (4H, dd, py), 5.43 (4H, s, CH2). C
NMR (500 MHz, CDCl3, 296 K) d 157.1, 148.1, 137.6, 137.1, 126.9, 117.1, 114.5, 51.02. MS
(HR-ESI): m/z 444.2078; C28H24N6 requires 444.2063.
1,3,5-Tris(di-2-pyridylaminomethyl)benzene (L4). This was prepared by an adaptation of a
published procedure.7 Di-2- pyridylamine (1.00 g, 5.84 mmol) and potassium hydroxide (1.31
g, 23.4 mmol) were stirred in DMSO (5 mL) for 1 h. 1,3,5- Tris(bromomethyl)benzene19 (0.675
6
Page 6 of 46

g, 1.89 mmol) was then added and the solution stirred for an additional 48 h. Water was added
dropwise until the solution turned cloudy. On standing, a yellow precipitate formed which was
filtered off, washed with water and recrystallized from an ethyl acetate–petroleum ether (bp 30–
60 οC) mixture. Yield, 0.583 g (49%); mp 158–160 οC. Found: C, 74.33; H, 5.07; N, 19.80. Calc.
for C39H33N9: C, 74.62; H, 5.30; N, 20.08%. 1H NMR (500 MHz, CDCl3, 296 K) d 8.20 (6H, d,
py), 7.37 (6H, t, py), 7.07 (3H, s, ph), 6.91 (6H, d, py), 6.78 (6H, dd, py), 5.34 (6H, s, CH2). 13C
NMR (125.76 MHz, CDCl3, 296 K) d 157.0, 148.0, 139.4, 137.0, 123.6, 116.9, 114.5, 51.18. MS
+
(HR-ESI): m/z 628.2937. C39H34N9 requires 628.2953.
1,2,4,5-Tetrakis(di-2-pyridylaminomethyl)benzene (L5). Di-2-pyridylamine (1.00 g, 5.84
mmol) and potassium hydroxide (1.31 g, 23.4 mmol) were stirred in DMSO for 1 h, 1,2,4,5-
tetrakis( bromomethyl)benzene (0.642 g, 1.89 mmol) was added and the solution stirred for a
further 48 h. Addition of water gave a yellow precipitate which was washed with water and then
recrystallized from a chloroform–petroleum ether mixture. Yield: 0.396 g (34%); m.p. 238–241
οC. Calc. for C50H42N12 .2H2O: C, 70.90; H, 5.47; N, 19.84. Found: C, 71.03; H, 5.36; N,
20.01%. ES-MS m/z = 811 [M+H]+. HRMS: calc. for [C50H43N12]+ 811.3729; found 811.3734.
1H NMR (CDCl3): d 8.16 (d, 8H, H1), 7.33 (Br, 8H, H3), 7.14 (s, 2H, H8), 6.84 (d, 8H, H4),
6.78 (dd, 8H, H2), 5.42 (s, 8H, H6). 13C NMR (CDCl3): d 156.8 (C5), 148.0 (C1), 137.0 (C3),
133.6 (C7), 123.6 (C8), 116.9 (C2), 114.5 (C4), 52.1(C6).
2.1.2. Re(I) complexes (C1 – C5) of the ligands: The general procedure [36] of the synthesis of
Re(I) complexes can be described as follows. A mixture of [Re(CO)5Br] (50 mg, 0.123 mmol)
and the corresponding 2,2′-dipyridylamine ligands (L1-L5) (0.123 mmol) were taken in 1:1 M
ratio in a two neck Schlenk flask fitted with a reflux condenser. The whole system was evacuated
to 2-3 times, nitrogen was purged and 20 ml of freshly distilled toluene was added by using
7
Page 7 of 46

syringe in the nitrogen atmosphere. The respective reaction mixture was stirred and heated up to
ο
60-70 C for 5-12 h and then allowed to cool to room temperature. Solvent was removed by
rotatory evaporator and the solid mixture was washed with hexane to remove the unreacted
starting precursor and the crude product was further purified by column chromatography in
neutral alumina by using hexane/ethyl acetate mixture in 1:2 ratio as an eluent to give pale
yellow or yellow products.
[Re(CO)3(1,2-bis-DPA)Br] (C1). Yield: 38 mg (76%); IR (KBr): ν 2020 (vs), 1909 (vs), 1892
(vs), 1601 (w), 1583 (w), 1468 (m), 1437 (w), 1425 (w), 1239 (w), 771 (w), 756 (w) cm-1; 1H
NMR (400 MHz, CDCl3, 25 οC): d 8.98 (d, 2H, J ¼ 1.2 Hz, CH(py)), 8.91 (t, 4H, CH(py)), 8.30
(d, 2H, J ¼ 4 Hz, CH(py)), 7.72 (t, 4H, CH(py)), 7.19 (s, 4H, CH(benzyl)), 7.13 (d, 4H, J ¼ 6.4
Hz, CH(py)), 5.24 (s, 4H, CH2) ppm; ESI-MS (m/z): 796.68 [M+]; UV-Vis {Acetonitrile, λmax
,
nm (ε/104 M-1 cm-1)}: 261 (0.95), 298 (1.02); Anal. Calc. for C31H24N6O3BrRe (794.67): C,
46.85; H, 3.04; N, 10.58. Found: C, 46.92; H, 3.33; N, 10.69%.
[Re(CO)3(1,3-bis-DPA)Br] (C2). Yield: 32 mg (64%); IR (KBr): ν 2020 (vs), 1908 (vs), 1884
(vs), 1601 (w), 1583 (w), 1468 (m), 1438 (w), 1376 (w), 1111 (w), 774 (w), 754 (w) cm-1; 1H
NMR (400 MHz, CDCl3, 25 οC): d 8.95 (d, 4H, J ¼ 5.6 Hz, CH(py)), 7.79 (s, 1H, CH(benzyl)),
7.69 (t, 4H, CH(py)), 7.40 (d, 2H, J ¼ 7.6 Hz, CH(benzyl)), 7.29 (t, 1H CH(benzyl)), 7.16 (d,
4H, J ¼ 8.0 Hz, CH(py)), 7.07 (t, 4H, CH(py)), 5.03 (s, 4H, CH2) ppm; ESI-MS (m/z): 797.21
[M+]; UV-Vis {Acetonitrile, λmax, nm (ε/104 M-1 cm-1)}: 210 (2.00), 254 (1.24), 298 (1.41); Anal.
Calc. for C31H24N6O3BrRe (794.67): C, 46.85; H, 3.04; N, 10.58. Found: C, 46.69; H, 3.23; N,
10.83%.
[Re(CO)3(1,4-bis-DPA)Br] (C3). Yield: 40 mg (80%); IR (KBr): ν 2021 (vs), 1909 (vs), 1892
(vs), 1601 (w), 1583 (w), 1467 (m), 1439 (w), 1424 (w), 1111 (w), 774 (w), 756 (w) cm-1; 1H
8
Page 8 of 46

ο
NMR (400 MHz, CDCl3, 25 C): d 8.92 (d, 2H, J ¼ 4.4 Hz, CH(py)), 8.25 (d, 2H, J ¼ 3.6 Hz,
CH(py)), 8.21 (d, 4H, J ¼ 4.0 Hz, CH(py)), 7.14 (d, 4H, J ¼ 5.6 Hz, CH(benzyl)), 7.44 (t, 4H,
CH(py)), 6.76 (t, 4H, CH(py)), 5.35 (s, 4H, CH2) ppm; ESI-MS (m/z): 796.43 [M+]; UV-Vis
{Acetonitrile, λmax, nm (ε/104 M-1 cm-1)}: 226 (0.49), 252 (0.33), 297 (0.34); Anal. Calc. for
C31H24N6O3BrRe (794.67): C, 46.85; H, 3.04; N, 10.58. Found: C, 47.02; H, 3.37; N, 10.79%.
[Re(CO)3(1,3,5-tris-DPA)Br] (C4). Yield: 41 mg (82%); IR (KBr): ν 2018 (vs), 1909 (vs),1889
(vs), 1597 (w), 1583 (w), 1467 (m), 1424 (m), 1375 (w), 1230 (w), 712 (w), 754 (w) cm-1; 1H
NMR (400 MHz, CDCl3, 25 οC): d 8.87 (d, 2H, J ¼ 4.0 Hz, CH(py)), 8.13 (t, 6H, CH(py)), 7.07
(s, 3H, CH(benzyl)), 6.90 (d, 4H, J ¼ 8.0 Hz, CH(py)), 6.85 (d, 6H, J ¼ 8.0 Hz, CH(py)), 6.73 (t,
6H, CH(py)), 5.34 (s, 6H, CH2) ppm; ESI-MS (m/z): 978.05 [M+];UV-Vis {Acetonitrile, λmax
,
nm (ε/104 M-1 cm-1)}:: 227 (0.57), 275 (0.39), 299 (0.46); Anal. Calc. for C42H33N9O3BrRe
(977.88): C, 51.59; H, 3.40; N, 12.89. Found: C, 51.81; H, 3.57; N, 13.03%.
Re(CO)3(1,2,4,5-tetrakis-DPA)Br] (C5). Yield: 35 mg (70%); IR (KBr): ν 3423 (w), 2019 (s),
1907 (s), 1892 (s), 1599 (w), 1584 (w), 1468 (m), 1425 (m), 1377 (w), 1237 (w), 1152 (w), 772
(w) cm-1; 1H NMR (400 MHz, CDCl3, 25 οC): d 8.31 (d, 2H, J ¼ 3.6 Hz, CH(py)), 8.20 (d, 6H, J
¼ 7.2 Hz, CH(py)), 7.51-7.49 (m, 8H, CH(py)), 7.29 (s, 2H, CH(benzyl)), 6.89-6.71 (m, 16H,
CH(py)), 5.50 (s, 8H, CH2) ppm; ESI-MS (m/z): 1183.88 [M+Na]+; UV-Vis {Acetonitrile λmax
,
nm (ε/104 M-1 cm-1)}: 228 (0.98), 298 (0.85), 387 (0.06); Anal. Calc. for C53H42N12O3BrRe
(1161.09): C, 54.82; H, 3.65; N, 14.48. Found: C, 55.07; H, 3.59; N, 14.83%.
2.2. Instruments and data analysis
Steady-state absorption and corrected fluorescence spectra were recorded on Perkin-
Elmer (model Lambda25) absorption spectrophotometer and Hitachi (model FL4500)
spectrofluorimeter, respectively. The final concentration of chromophores was adjusted at ca. 10
9
Page 9 of 46

µM in the solutions used for spectral measurement. In fluorescence emission and excitation
spectra measurements, 5/5 nm band pass was used in both the excitation and emission sides.
Fluorescence quantum yields ( ) were calculated by comparing the total fluorescence intensity
f
(F) under the whole fluorescence spectral range with that of quinine bisulfate in 0.5M H2SO4
(
S = 0.546) [37, 38] by using Eq. (1).
f
s
F i 1−10− A ni
2
φif
=
φfs
.
.
.(
)
(1)
i
F s
ns
1−10−A
Where, Ai and As are the optical density of the sample and standard, respectively, and ni is the
refractive index of solvent at 298 K. The relative experimental error of the measured quantum
yield was estimated within ±10%.
The fluorescence decay curves were obtained by using 295 nm LED excitation in a Pico
Master time correlated single photon counting (TCSPC) lifetime apparatus (PM-3) supplied by
Photon Technology International Inc. (PTI). The instrument response function (IRF) was
obtained by using a dilute colloidal suspension of dried non-dairy coffee whitener. The
experimentally obtained fluorescence decay traces I(t), collected at the magic angle (54.70) to
eliminate any contribution from the anisotropy decay, were expressed as a sum of exponentials
(Eq. 2) and analyzed by non–linear least–square iterative convolution method based on
Lavenberg–Marquardt algorithm [39] as implemented in the data analysis software (Felix GX
version 4.0.3) from PTI
ꢀ ꢂ = ꢅ ꢄꢅ expꢁꢆꢈꢇꢃ
(2)
ꢁ ꢃ
∑
ꢉ
Where, αi is the amplitude of the ith component associated with fluorescence lifetime τi such that
αi = 100. The reliability of fitting was checked by numerical value of reduced chi- square (χ2),
Durbin–Watson (DW) parameter and also by visual inspection of residual distribution in the
10
Page 10 of 46

whole fitting range. All experiments were carried out at ambient temperature of 298 K, and the
other spectral parameters like average fluorescence lifetime (<τ>), radiative and total non-
radiative rate constants (kr and Σknr, respectively) were calculated by using equations (3), (4) and
(5), respectively.
ꢌ
∑
ꢋ ꢈ
ꢉ ꢉ
ꢉ
< ꢊ > =
(3)
(4)
(5)
∑
ꢋ ꢈ
ꢉ
ꢉ
ꢉ
ɸ
ꢏ
ꢍꢎ = ꢐꢈꢑ
ꢓꢆ
φ
ꢏ
ꢒꢎ
∑
ꢍ
=
ꢐꢈꢑ
2.3. Density functional theory (DFT) calculation.
The geometries of the ligands (L1-L5) and their corresponding Re(I) complexes (C1-C5)
were optimized using DFT based B3LYP method [40] in conjugation with 6-31G(d,p) basis set
for the lighter atoms (C, H, N and O) and LANL2DZ basis set for the heavier atoms (Re and Br).
The 6-31G(d,p) is a standard polarized basis set, which adds d function on heavier atoms and p
function on lighter atoms; whereas, LANL2DZ is a pseudo potential basis set frequently used for
post third row atoms [41, 42]. Harmonic frequency calculations were carried out at the same
level to characterize the stationary points to ensure that the optimized geometries are minima on
the potential energy surface (PES). The Time dependent-DFT (TD-DFT) method [43] was used
to calculate the electronic absorption spectra of the complexes. In order to incorporate the effect
of the solvent around the molecule, the Polarizable Continuum Model (PCM) [44] was employed
in TD-DFT calculations. The percentage composition of the molecular orbital analysis was
carried out by Chemissian software package [45]. All the electronic structure calculations were
carried out using Gaussian09 suite of program [46].
11
Page 11 of 46

3. Results and Discussion
3.1. Steady state spectra of ligands and Re (I) complexes.
Representative electronic absorption spectra of ligands and it’s Re(I) complexes are
illustrated in figure 1; while the corresponding plots for all the systems are given in figure S1 in
supplementary section. The main intense absorption band (ε ~ 104 M-1 cm-1) of the ligands is
around 300 nm in DMSO and shifts to 304 – 309 nm range in 98% (v/v) water (XWater =0.99).
Also, in the case of multi-chromophoric structures like trimer and tetramer, a lower energy broad
peak is introduced around 375 - 394 nm due to the spin allowed intramolecular inter ligand ππ*
transition. With increasing mole fraction of water, vibrational structure in this absorption band
becomes prominent (Figure S1). The absorption behavior of the complexes is almost similar to
the respective ligands. The main intense band again appears at around 300 nm. However, in case
of Re(I) complexes, the structural features as well as intensity of the low energy absorption band
is relatively less in comparison with the ligands, due to ligand to metal charge transfer as
confirmed by theoretical calculation result discussed later.
All the ligands give emission at around 360 nm when excited at 300 nm absorption in
pure DMSO. However, on further increasing the mole fraction of water, the emission peak gets
shifted towards 410 nm with substantial broadening in spectral width (figure 2a). These results
indicate that increasing the water content in the solvent mixture produces a new “solvent
relaxed” emissive state which is stabilized by ca. 3250 cm-1 than the corresponding Frank-
Condon excited state in pure DMSO. Interestingly, in the mixtures containing very low DMSO
content, the stabilization of the excited state is not apparent; in fact, the emissive state becomes
less stable by ca. 1000 cm-1 for L1. Similar observation is also observed for other ligands, but to
a different extent. This observation, although seems to be contradictory in nature, can be
12
Page 12 of 46

explained by the recent studies of Bagchi et. al. [47] and subsequent experimental findings by
Sarkar et. al. in different systems [48, 49]. Based on simulation studies, Bagchi and coworkers
have proposed that at low mole fraction of DMSO, an abrupt increase in the hydrophobicity of
the DMSO−water mixture occurs, which destabilizes polar solute to a greater extent. Therefore,
the shift in fluorescence spectral peak position in high water content solution is considered to be
due to the stabilization of polar excited state in the solvent cage of water. Stabilization of the
polar excited state resulting highly solvent dependent luminescence from Ru(II) complexes is
already reported in the literature [50]. The polar nature of the emissive state in the present
systems is further confirmed by critically reviewing the spectral shift (inset of figure 2a). For
example, the destabilization at the region characterized by XDMSO < 0.1 is only 250 cm-1 for L3
in contrast to ca. 1000 cm-1 of the corresponding parameter for L1. This observation is consistent
with the polarity of the ligands (L3 is least polar among the bi-chromophoric compounds;
whereas, the polarity of L1 is expected to be highest) and corroborates nicely with the DFT
calculation results (see below).
Furthermore, in case of higher order ligands like trimer (L4) and tetramer (L5), the
emission intensity is very weak (see below for a comparison on relative fluorescence yield for
different ligands and figure S1) and the spectral profile consists of two distinctly visible peaks;
one corresponds to the 360 nm as observed in L1 – L3 and the other very broad peak within
400~550 nm region, particularly at higher water content solution. Interestingly, this broad
emission peak shows striking resemblance with the emission spectrum obtained by exciting the
sample at low energy absorption band (ca. 375 nm) as shown in the inset of figure 2b. Therefore,
the long wavelength absorption (375 ~ 394 nm) and the corresponding broad emission (400 ~
550 nm) can be assigned to a new emitting state or species, which is more facile to form in case
13
Page 13 of 46

of multi-chromophoric systems with excess water content in solvent mixture. A possible
explanation on the origin of these spectral features can be drawn from the conformational
analysis of the ligands. A significant steric repulsion between the adjacent pyridylaminomethyl
units is expected in L1 and/or L5 leading to a twisted conformation in contrast to L2, L3 and L4,
where the chromophoric sub-units are well separated. However, as the low-energy absorption
and/or emission bands are observed only in case of higher order ligands, the importance of
twisting in the photophysical behavior can be ruled out. Rather, the spin allowed intramolecular
inter-ligand type of interaction, hypothesized earlier, becomes more pertinent in higher order
systems studied here, particularly in high water content (less viscous) solvent mixtures due to
greater flexibility of the branched chromophores.
The general trend of fluorescence for mono-metallic Re (I) complexes (C1 – C5) in
different DMSO-water mixtures are similar to that observed for the corresponding ligands (L1 –
L5). A representative plot for C4 is shown in figure 2b. The only notable difference is
significantly less fluorescence spectral shift observed in case of the complexes relative to their
ligand counterpart (figure S1). For example, the fluorescence peak shifts from 360 nm in pure
DMSO to 393 nm only (∆E = ~2330 cm-1) in 40% (v/v) DMSO-water mixture in case of C1.
Therefore, the emissive states of the metal complexes are considered to be significantly less
polar in comparison with the ligands. DFT calculation results for the dipole moment of the
investigated systems (see below) indeed confirm this prediction.
The emission properties of all the ligands and the complexes were checked with varying
excitation wavelength within the whole absorption profile. Representative plots for all the
systems are shown in supplementary section (figure S2). Except certain changes in the intensity,
the emission spectral profile remains invariant in all the cases suggesting that the primary
14
Page 14 of 46

photophysical behavior is mostly due to the ligands only, even in case of the metal complexes.
Earlier report on the non-luminescent Re complexes of several di(2-pyridyl)amine based ligands
3
was ascribed mainly due to the metal centered excited state or phosphorescence from MLCT
state [51]. However, almost similar spectral behavior of the metal complexes with their
corresponding ligand counterpart observed in the present study emphasizes mainly ligand
centered absorption (even in presence of metal). This is probably due to very weak molar
absorption coefficient of the MLCT state in comparison with very strong π*←π ligand
absorption.
3.2. Quantum yield of fluorescence of the ligands and Re(I) complexes.
Fluorescence quantum yields (φ ) of all the systems were measured in different solvent
f
mixtures with varying water content. The representative spectral data of all the investigated
systems in some selected solvent systems are collected in table 1; while, the detailed result of the
individual systems in different solvent mixtures are given in supplementary section (table 1S and
2S). In general, φ of the ligands are at least one order of magnitude higher than that of the
f
corresponding complexes, which further emphasize on the dπ(Re)-π* type (see
DFT calculation results also) metal to ligand transition in the complexes. It is further to be noted
that φ of L3 is highest among all the ligands; while the same is true for C3 among all the
f
complexes. It is found to be the lowest in the cases L2 (or C2) and L5 (or C5). These
observations are consistent with the assumption of inter-ligand cross talk between the
neighboring di-2-pyridylaminomethyl groups in the multi-chomophoric systems mentioned
above. Similar observations were also noted in the cases of polyphenylene dendrimers with
multiple peryleneimide chromophores in the rim [52]. The intramolecular energy hopping in the
multi-chromophoric dendritic structures were shown to occur via Förster mechanism within
15
Page 15 of 46

ultrafast time scale of 100~200 ps [53]. The efficiency of energy transfer depends critically on
the spatial arrangement of interacting chromophores, i.e. the geometric distribution between the
transition dipole moments and also the distance between the donor and acceptor pair, the so
called critical Förster distance, R0 [54]. The exact nature of the transition dipole moment
distribution between the donor-acceptor pair is unknown at this point for any of the investigated
systems. Therefore, quantitative comparison of the excitation energy transfer efficiency among
different systems is not possible. However, a qualitative description for the observed difference
in fluorescence yield can be put forward by comparing the inter-chromophoric distances in
different ligands with an assumption of random dipole moment distribution. The distance
between two amino-methyl-pyridyl chromophores in L1, L2 and L3 are 3.42, 4.95 and 7.53 Å,
respectively [34]. Among these, the efficiency of energy transfer is highest in L2 (lowest φ );
f
therefore, the critical distance (R0) for efficient energy transfer can be assumed to be ca. 5.0 Å.
The para substituted ligand (L3) can be considered as a model system, where inter-chromophoric
interaction is the lowest; therefore showing highest φ among the lot. On the other hand, the
f
distance between the two chomophores in L1 seems to be too short (R<<R0), so the efficiency of
energy transfer is very less. This may explain the observed φ in the case of L1, which is quite
f
close to L3 (figure 3a). Interestingly, the φ value in case of L4 is quite close to L2. From the X-
f
ray crystal structure data [34], it is apparent that although this system contains three 1,3
substituted chromophores, only one pair of them are in the range of R0 (R = 4.9Å); the other two
distances (6.7 and 7.4 Å, respectively) are very large and therefore are not expected to contribute
in the energy transfer process significantly.
Further, the variation in φ with solvent composition shows some peculiar behavior. With
f
increasing water content in solution, the quantum yield of both the ligands as well as complexes
16
Page 16 of 46

increases initially. This is consistent with the stabilization of relatively more polar emitting state
with increasing polarity of the solvent. However, the magnitude of φ increases only upto certain
f
mole fraction of water, typically Xw ~ 0.7 for ligands and 0.8 in case of the complexes (figure 3).
It is well known from the earlier studies that at low DMSO (or higher water) content, the solution
mixture forms several hydrogen bonded water-DMSO complexes [47, 48], and therefore, the
local water structure near the probe becomes more hydrophobic in nature and cannot stabilize the
polar emissive species.
3.3. Time resolved fluorescence studies on the ligands and Re(I) complexes.
Time resolved fluorescence decay behavior of all the ligand and complexes were
measured by monitoring the fluorescence at their respective emission wavelength in several
DMSO-water mixtures. At least two exponential decay model in Eq. 2 was necessary to fit all the
data with acceptable statistical parameters as evidenced by some representative fluorescence
decay fittings of L5 and C3 shown in figure 4. While τav, the average fluorescence decay time
(calculated using Eq. 3), of individual systems are listed in table 1, the results of complete decay
analysis of all the systems are shown in supplementary section (tables 3S and 4S). In general, the
magnitude of τav for the metal complexes are higher than the corresponding ligands. The
calculated values of radiative (kr) and sum of total non-radiative (Σknr) rate constants for
different systems are also shown in table 1.
Time-resolved fluorescence decay shows interesting modulation with solvent
composition for all of the investigated systems. Representative examples are shown in figure 5
for L3 (para substituted dimer) and L4 (1,3,5 substituted trimer). The correosponding plot for the
other systems are shown in the supplementary section (figure S3). It is clear that the decay
pattern changes completely with varying water content in the solution. Furthermore, the trend in
17
Page 17 of 46

variation of the deacay profile is almost opposite in case of L4 while comparing with L3. To
understand more about solvent dependent fluorescence decay behavior of all the investigated
systems, global anaysis of the fluorescence decay data was performed for individual systems in
different solvent mixtures. Two lifetimes related with the fast and slow decay components
ranging within τ(fast) = 0.8~1.7 ns and τ(slow) = 2.9~6.8 ns are observed for the ligands; while
the corresponding values for the complexes are found to be within 1.1~2.6 and 5.2~9.4 ns,
respectively. To verify the possible involvement of phosphorescence decay associated with long
lifetime, particularly in case of Re complexes, all the luminescence measurements were repeated
under deoxygenation condition by saturating the solution with argon gas. However, no difference
in the luminescence spectral profile as well as in the time resolved decay behavior taken over
whole emission spectral range were observed in any of the systems (supplement figure S4). Also,
3
typical phosphorescence lifetime from the MLCT state of Ru(II) and Ir(III) complexes are
known to be within a few hundreds of picosecond to tens of microsecond time scale [55, 56].
Furthermore, the variation in relative amplitude of the long decay component with solvent
composition show similar pattern for both the ligands and the complexes in the present study
(see below). Therefore, the decay component ranging within 5.2~9.4 ns in case of metal
complexes is considered not to be due to any phosphorescent state; however, the possibility of
inter system crossing and thermally activated delayed fluorescence (TADF) can not be ruled out
at this point. This is more relevant as some of the recent reports suggested very efficient TADF
in metal complexes with intra-ligand charge transfer properties [57, 58]. Interestingly, the
relative amplitude associated with the fast decay component, a(fast), is always predominant
(>90%) in pure DMSO or in the DMSO-water mixture with 98% (v/v) water content. However,
interesting modulation in relative population is observed at low DMSO content of the solution
18
Page 18 of 46

(Xwater = 0.8), particularly for the ligands. In all the cases, the contribution from the slow decay
component, a(slow), increases considerably (upto 30~40%) under this condition. Some of the
representative plots are shown in figure 6. These results indicate that the species associated the
faster decay component is considerably polar in nature and therefore, predominates in highly
polar media. However, in the solvent with XDMSO = 0.8, where the hydrophobic nature of the
solvent system becomes important, the relatively non-polar species responsible for the slow
decay component also contributes significantly in the fluorescence emission. This observation is
also consistent with the fact that fluorescence emission spectrum in almost all the cases show
significant broadening in the solution mixture with Xwater = 0.8 (figure 2) discussed above. It is
noted here that a complete picture on the solvent modulation of fluoescing species can be
obtained by constructing the decay associated spectra of both these lifetime components at
different wavelengths in the whole emission spectral profile under varying solvent composition.
3.4. DFT calculation results on the ligands and Re(I) complexes.
3.4.1. Optimized Geometries. The geometries of all the metal complexes are found to be closed
shell structures and no parameter is kept constrained during their optimization. It should be noted
that, in a previous work we have already reported the theoretical studies for the complexes C2,
C4 and C5 [36]. In this present work, we report various theoretical results for all the ligands and
their corresponding metal complexes to substantiate the experimental results discussed in the
previous sections. It has been already observed that the various geometric parameters such as
bond lengths (r) and bond or dihedral angles (θ or δ, respectively) for the complexes are in well
agreement with the single crystal X-ray structural data which accounts for the reliability of the
theoretical method (B3LYP/6-31G(d,p)/LANL2DZ) used for the present work. The distance
between two amino-methyl-pyridyl chromophores i.e, distance from one methelene carbon to the
19
Page 19 of 46

other methelene carbon in L1, L2 and L3 are 3.02, 5.05 and 5.86 Å, respectively which is in
good agreement with experimental data described above. The ground state dipole moments for
the ligands (L1-L5) computed at the B3LYP/6-31G(d,p) level of theory are found to be 4.74,
4.80, 4.39, 5.60 and 1.62 D, respectively. As expected, among all the bis-complexes, L3 has the
lowest dipole moment than the other two ligands. This difference can be accounted for the higher
symmetry in the geometry of L3 molecule. Similar effect can be observed for L5 as well. A
schematic representation of the ligands is shown in scheme - I. It should be noted that the Re
metal coordinates to the ligand through N2 and N3 of fragment A in the complexes C1 – C5.
Almost for all the ligands, θ(C1C7N1) is found to be 116º and negligible variation has been
observed for the metal complexes. On the other hand, δ(C1C7N1C9) for ligands L1 - L4 ranges
between 95º~98º; whereas, for L4, the corresponding value of 62º has been predicted. On
complex formation, the same dihedral angle decreases to 88º, 48º, 42º and 41º for C1, C2, C4
and C5; whereas, for C3 it increases to 162º. The induced planarity of the pyridylaminomethyl
ligand with the rigid benzene core on complex formation ensures MLCT type of transition
mentioned earlier. Interestingly, δ(C1C7N1C10) for L1 - L4 ranges between -78º ~ -83º;
whereas for L5, an angle of -139º has been predicted. On complex formation, this angle varies
within -150º to -163º for C2, C4 and C5; whereas for C1 and C3, a value of -84º and -37º has
been predicted. The bond distance r(N2-N3) in ligands L1-L4 ranges between 3.02~3.05 Å;
whereas for L5, a slight higher value of 3.76 Å has been observed. These distances reduce to
2.82, 2.87, 2.92, 2.93 and 2.96 Å, respectively for C1-C5 on complex formation.
The charges on the individual atoms for ligands and the metal complexes obtained from
Natural Bond Orbital (NBO) analysis [59] are listed in table 5S. The charges on the Re atom for
the metal complexes ranges between -0.9615e and -0.9963 which are lower than the formal
20
Page 20 of 46

charge of Re i.e. +1. On the other hand, the NBO charges on N1, N2 and N3 atom for L1-L3 are
found to be -0.473e, -0.453e and -0.490 e. On complexation, N1, N2 and N3 loses its negative
charge partially and attains a value of -0.448, -0.412 and -0.425e; -0.467, -0.422 and -0.423e;
and -0.446, -0.410 and -0.416 e for complexes C1, C2 and C3, respectively. A similar decrease
in N charges has been observed for C4 and C5 as well, except for a slight increase in N3 charge
in complex C5 (as indicated in Table 5S). All these results suggest that there is a charge transfer
(CT) from ligand to Re(I) metal (LMCT) on complex formation.
3.4.2. Nature of Frontier Molecular Orbitals. The HOMO-LUMO energy gap is known to be a
good measure for the absorption spectral peak position of a molecule. The HOMO-LUMO
energy gap (∆E) for all the ligands are found to range between 4.38 ~ 4.58 eV (corresponding to
283 ~270 nm). On complex formation, the corresponding value calculated for C1-C5 reduces to
3.78, 3.75, 3.67, 3.53 and 3.85 eV, respectively (328 ~ 322 nm). The frontier molecular orbital
diagram as well as the composition of molecular orbital analysis of all the ligands suggests that
the HOMO (nearly 90%) is mainly located in the pyridylamine moiety (fragment A, scheme I);
whereas, LUMO is located on the other pyridylamine moiety (fragment A1 for L1, A2 for L2, A3
for L3 and A2 for L4). This pattern is consistent for all the ligands except for L5, where about
70% of the HOMO is located on fragment A but a small percentage of HOMO (15% and 16%) is
spread over A5 and A2 as well (figure S5). It is to be noted that this complete reversal of electron
density on excitation from one fragment to the other confirms the possibility of inter-ligand cross
talk in the excited state in a particular molecule, hypothesized earlier from the fluorescence
spectral data. From the % composition of the molecular orbital analysis for metal complexes, it
can be concluded that HOMO (ranging between 40-63 %) is mostly located on the Br atom for
complexes C1, C2, C3 and C5; whereas for complex C4, it is located on the ligand itself (figure
21
Page 21 of 46

S6). Unlike other complexes, almost 30% of the HOMO for C5 is located on the meta substituted
A2 fragment. This pattern is missing in other complexes. On the other hand, more than 90% of
LUMO is located on fragment A of the ligand for all the complexes.
The electronic absorption spectra were calculated using the TD-DFT method in two
different solvents (DMSO and H2O) employing PCM model, which essentially deals the solvent
as a dielectric continuum. The calculated and the experimental absorption data, HOMO-LUMO
energy gaps, oscillator strength and the character of electronic transitions are listed in table 2.
Some of the representative figures are also in supplementary section (figure S7). In agreement
with the experimental results, theoretical calculations also predict an intense absorption band of
the ligands (L1-L5) at around 290 nm in DMSO. However, no significant shift in absorption
peak is observed in aqueous medium within the PCM framework. Therefore, it can be argued
that experimentally observed long wavelength absorption band in aqueous medium (particularly
for multi-chromophoric system) is not only originated due to the dielectric properties of the
solvent. Rather, the presence of more than one chromophore in close vicinity for highly
substituted systems and more flexibility in the side chains in water medium (in comparison with
highly viscous DMSO solvent) appear to be the most probable reason for this new absorption, as
discussed in the previous section.
The HOMO-LUMO transitions for all the complexes follow the same pattern i.e.
pπ(Br)→π (L) transition except for complex C4, where an ILCT (π→π ) transition has been
observed. Few MLCT transitions at around 288~297 nm have also been observed for C1, C2 and
C3 complexes which corresponds to d(π)Re→π (L). It should be noted that no such MLCT
transitions has been observed for the other two complexes (C4 and C5) containing the tris and
tetra (di-2-pyridylaminomethyl)benzene ligands. On the other hand, for C4 and C5, most of the
22
Page 22 of 46

transitions around 285~293 nm corresponds to ILCT transitions. The absorption behavior of the
complexes is almost the same as their respective ligands. All these transitions occurring around
285~297 nm are in well agreement with the experimentally observed results.
Conclusions
The photophysical behavior and excited state relaxation process of strategically
substituted flexible di-2-pyridylaminomethyl chromophores on a rigid benzene framework and
their corresponding mono-metallic Re(I) complexes were investigated by steady state and time-
resolved fluorescence spectroscopy in conjunction with density functional theory calculation. An
abrupt break in the transition of fluorescence parameters at low DMSO concentration in DMSO-
water binary solvent mixture is apparent in all the systems and explained on the basis of
unavailability of water hydrogen bonded network structure at XDMSO = 0.1 ~0.3. Experimental as
well as theoretical calculation results predict spin allowed inter-chromoric interaction and
complete reversal in accumulation of electron density on excitation in highly substituted ligand
systems, particularly in trimer and tetramer. The results are interesting and can lead towards
designing a dendritic architecture similar to artificial photosynthetic antenna system by suitably
placing a trapping site along with the di-2-pyridylaminomethyl chromophores as peripheral
flexible subunit on the benzene core.
Acknowledgements. The authros acknowledge the support from the Dept. of Science &
Technology (DST), Govt. of India to the Chemistry Department through FIST program
(SR/FST/CSI-194/2008). Computational facilities provided by the Computer Center, NEHU is
gratefully acknowledged.
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Supporting Information available. Supporting data associated with this manuscript is
available online.
24
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